1. Field of the Invention
Exemplary embodiments of the present invention are related to range gating within a Light Detection and Ranging (LIDAR) system. More particularly, exemplary embodiments are related to the LIDAR systems in which timing of a pulsed laser beam and a scanner is coordinated to define a range gated signal according to a desired range gate for performing measurement of atmospheric data products.
2. Summary of the Prior Art
U.S. Pat. No. 7,315,377 to Holland et al. discloses a system for remote sensing and analyzing spectral properties of a target or chemical. Holland uses a scanner to illuminate different detectors, which are not scanning to perform any range gating. This system is passive, and its scan views all ranges within the field of view simultaneously.
U.S. Pat. No. 5,231,401 to Kaman et al. discloses an imaging system for forming an image of an object with a scanning mirror and a multiple camera assembly. Kaman uses an object plane scanner with a gated detector. An intensified charge-coupled device (CCD) is used to provide the necessary range gating. The moving mirrors are used to scan the sensor's field of regard and not range gate.
U.S. Pat. No. 5,831,719 to Berg et al. discloses a laser scanner for measuring spatial properties of objects. Berg employs an object plane scanner.
U.S. Pat. No. 7,534,984 to Gleckler shows an electronic means to scan the beam, using a rotating polygon mirror and a Micro-Electro-Mechanical System (MEMS) mirror as methods of implementing a streaking camera.
U.S. Pat. No. 5,006,721 to Cameron et al. discloses a LIDAR scanner incorporating a polygonal mirror. Cameron describes an object plane scanner that scans the field of view of the transmitted laser beam and the field of view of the receive telescope across the scene.
The output of a Fabry-Perot interferometer is an image that is modified by the atmospheric data products identified above. In the LIDAR system, a detector converts the backscattered light in the form of an image to an electrical signal that is processed to produce the atmospheric data products. These LIDAR systems make measurements in a single range bin defined by the overlap of the transmitted laser beam and the receiver FOV.
In some cases, it is desirable to make measurements at different ranges simultaneously. For example, one may wish to make measurements in a 20 meter long volume of air at ranges of 150 and 250 meters from the LIDAR system. The 20 meter long volume is often referred to as a range bin. Two methods for implementing measurements of rang bins at different ranges have been disclosed previously.
In one method that has been published as patent application WO 2011/016892A2, the parallax between the transmitted beam and the received beam allows for atmospheric data product measurements to be simultaneously made at different ranges with different range bins. The imaging characteristic of this design allows one to use CCD or similar detectors which are advantageous in that they allow one to make simultaneous measurements at different ranges. The CCD or similar detector allows one to integrate multiple pulses into a single measurement to improve the measurement precision. This technique may also be used with a continuous wave laser. There is however a limitation in that the separation between the source and receiver can become quite large if range bins on the order of 20 meters at ranges of 100's of meters is desired.
A second method that one may use for obtaining range binned atmospheric data product measurements has been described in patent application WO 2010/124038A2, where a micro-mirror device is used to segment the Fabry-Perot image into independent patterns that enable one to use a higher bandwidth detector such as a photo multiplier tube (PMT). In this case, the range bins are defined by how long the signal from the PMT is integrated, and time of flight is used to establish the range from the sensor. For each of the atmospheric data products, one or more segmentations are required to produce independent segmented images for computation of each of the atmospheric data products.
As shown in FIG. 1, a simplified block diagram of a standard LIDAR system 100 is illustrated. While the transmitted beam 110 and the receive telescope field-of-view (FOV) 112 are shown in a configuration where transmit and receive optical axes are offset, there are cases where the transmit and receive optical axes could be common.
In the case shown, the CCD camera 114 will observe the entire interaction region during a CCD frame time. In practice the CCD exposure may integrate many laser pulses to obtain a signal strong enough to make the atmospheric data product measurements. As mentioned above, there are instances where one would like to make measurements over pre-determined range intervals or range bins at different ranges from the sensor. In effect, one would like to place a “shutter” in the path of the Receive Fiber Optic 118 to “range gate” the atmospheric backscatter observed.
For example, Q-switched lasers that are often used in LIDARs have laser pulses that are on the order of 10 to 20 ns in duration which translates to an out and back range of 1.5 to 3 meters. The pulse width puts a limit on the minimum range bin size. However, in the measurement of atmospheric data products, a range bin on the order of 20 to 50 meters is more appropriate. A range bin of 20 to 50 meters corresponds to integration times of 133 to 333 ns. Since most shutters have aperture times on the order of milliseconds they are not adequate for the task at hand.